And most of that is dust, rather than pollution.

Atmospheric aerosols, tiny solid particles such as soot and dust, affect not only air quality—and therefore human health—but also the climate. Aerosols absorb and scatter both incoming and outgoing radiation, directly affecting air temperature, and also indirectly influence climate by influencing cloud formation.

Unfortunately, the contribution from aerosols is one of the more uncertain aspects of global climate models, in part because of their complicated movement: they can travel thousands of miles with the wind, affecting countries and even continents far from their source. A paper published in this week's issue of Science shows that the volume of these traveling particles can actually rival the amount produced locally. This means that regulations on local emissions will only help fix part of the problem.

The team behind the paper, made of up scientists from NASA’s Goddard Space Flight Center and the University of Maryland, studied the movement of aerosols over North America using two instruments aboard NASA satellites: the Moderate-resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). MODIS measures the transparency of atmosphere as a way to determine the amount of aerosol present—more particles means less transparent. In addition, it can distinguish between and determine the fractions of various aerosols like dust, soot, and sea salt.

CALIOP complements these measurements by providing the vertical profile of different aerosols throughout the atmosphere over time. This allows the scientists to measure seasonal variation. For example, they found that spring is the most active season because extratropical cyclones (otherwise known as “depressions” or “lows”) and the mid-latitude Westerlies push aerosols over the Pacific to North America. In general, though, this transport occurs year-round.

The combined MODIS–CALIOP data, referred to as MODIOP by the authors (which, to their credit, sounds better than CALIOPODIS) showed that 140 teragrams, or over 108 tons, of dust left East Asia and 56 teragrams reached the North American west coast in 2005. (The missing 84 teragrams either fell into the Pacific or reached the Arctic.) By comparison, four teragrams of combustion aerosols arrived via the same route, while another four teragrams of dust came from the Sahara over the Atlantic Ocean.

For a little perspective, the total amount of particulate matter that is produced or emitted in North America is 69 teragrams—just a little more than the amount arriving over the Pacific! In other words, not only does the amount of imported aerosols from Asia match that produced domestically, but most of it is dust, rather than combustion-generated particles as conventional wisdom would suggest.

The authors also included some uncertainty estimates for their calculations of aerosol mass: 55 to 100 percent. This might seem high, but it is an improvement over the uncertainty in previous studies—by at least a factor of two. Also, the authors argue that the qualitative conclusions of this study are more important than the absolute numbers.

While the paper has implications for improving the aerosol transport portions of climate models, the more important takeaway message is arguably in policy: focusing on curbing particulate emissions on a national or regional basis only tackles half the problem.

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Kyle Niemeyer
Kyle is a science writer for Ars Technica. He is a postdoctoral scholar at Oregon State University and has a Ph.D. in mechanical engineering from Case Western Reserve University. Kyle's research focuses on combustion modeling. Emailkyleniemeyer.ars@gmail.com//Twitter@kyle_niemeyer